FIG. 8 is a block diagram that shows an example of a conventional PLL circuit shown in Japanese Patent Application Laid-open Publication No. HEI 9-153799. As shown in FIG. 8, this PLL circuit 1000 includes a phase comparator 1100, a loop filter 1200, a voltage controlled oscillation (VCO) circuit 1300, a frequency-dividing circuit 1400, and a frequency detecting circuit 1500. A horizontal synchronizing signal fH and a reference signal fr from the frequency-dividing circuit 1400 are input into the phase comparator 1100. The phase comparator 1100 then supplies detected signals to the loop filter 1200. A control signal VL output from the loop filter 1200 is input into the VCO oscillation circuit 1300. The VCO oscillation circuit 1300 then outputs a frequency signal nfH that is n times of the frequency of the horizontal synchronizing signal fH. Further, the horizontal synchronizing signal fH and a clock signal CL are input into the frequency detecting circuit 1500. The frequency detecting circuit 1500 outputs a characteristics change-over signal MSn for changing over the oscillation characteristics of the VCO oscillation circuit 1300.
FIG. 9 shows the operation of the VCO oscillation circuit 1300. Based on the oscillation characteristics changed over by the VCO oscillation circuit, the VCO oscillation circuit 1300 carries out an oscillation operation in a predetermined frequency according to the control signal VL output from the loop filter 1200.
Accordingly, for obtaining oscillation characteristics over a wide range, the VCO oscillation circuit 1300 is operated in a frequency area as shown by a broken line in FIG. 9. Thus, a wider dynamic range is realized.
According to the above-described conventional PLL circuit, however, when there is an increase in the oscillation characteristics, it is necessary to expand the operation frequency area of the VCO oscillation circuit along with this increase. Thus, the operation becomes unstable, and it becomes difficult to automatically select necessary oscillation characteristics.